The present invention generally relates to an engine combustion condition detecting apparatus equipped with a malfunction (failure) diagnosing apparatus. More specifically, the present invention is directed to such an engine combustion condition detecting apparatus equipped with a malfunction diagnosing apparatus, capable of preventing an erroneous diagnosis of engine combustion conditions caused by malfunctions of this engine combustion condition detecting apparatus.
To detect a combustion condition of an engine, it is important to judge as to whether this combustion condition is under normal condition, or under misfire condition. Conventionally, as an apparatus for detecting a combustion condition, a combustion pressure sensor is used to detect pressure within a combustion chamber. However, since this combustion pressure sensor is expensive, this sensor could not be mounted on each of cylinders due to high cost reason. Accordingly, this combustion pressure sensor is installed only on a specific cylinder to measure combustion pressure thereof. Then, the combustion conditions of the engine are detected by this measured combustion pressure.
Also, to judge an occurrence of misfire in an engine, a torque variation of this engine is acquired as a change in the number of revolutions, and a fluctuation in the engine speed is detected. In this case, an error is produced in the variation of the engine speed unless disturbance is removed based on drive condition. Thus, there is a risk that the fluctuation is erroneously detected.
Under such a circumstance, the following detecting technique has been proposed in which since the combustion conditions within the combustion chambers of the engine are directly detected, the combustion conditions of the engine and the misfire condition are detected. In other words, this conventional detecting technique is performed by measuring the amount of ions contained in the combustion gas within the combustion chambers by way of the ion current detecting means. The combustion conditions of the engine can be directly detected by using this conventional detecting technique without detecting the change of the engine speed, while considering the disturbance and the combustion pressure sensor.
Japanese patent application Un-examined Publication No. JP-A-4-308360 discloses the engine combustion condition detecting apparatus with employment of the above-explained ion current detecting means.
On the other hand, this proposed combustion condition detecting apparatus is arranged by the ion current detecting circuit for detecting the ion currents produced by the combustion within the respective cylinders of the engine, and also the judging means for judging as to whether the internal condition of the cylinder during the ignition operation is under normal combustion, or under misfire condition by using the ion current signal outputted from this ion current detecting circuit. This ion current detecting circuit means is inserted into the conventional ignition circuit. There is no problem when this ion current detecting circuit means is operated under normal condition, namely without under malfunction condition. However, when this circuit is brought into the malfunction condition, even when the ignition operation is performed under normal state, such an erroneous judgement is made in the case that the misfire happens to occur. Moreover, there is another problem that which circuit portion of the ion current detecting circuit means is under malfunction condition.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide a combustion condition detecting apparatus equipped with a malfunction diagnosing apparatus capable of diagnosing malfunction of a detecting apparatus such as an ion current detecting circuit for detecting a combustion condition of an engine in a stage for detecting combustion conditions.